CHEMICAL PROPERTIES OF FUSED QUARTZ / SILICA
Fused quartz (transparent) and fused silica (translucent) are amorphous products resulting from high temperature treatment of naturally occurring quartz crystals or sand (SiO2). Fused quartz has a nominal purity of 99.995%, and fused silica 99.90%. Fused quartz and silica are attacked by phosphoric acid at temperatures in the range of 150 degrees C or higher, and by hydrofluoric acid at all temperatures.
PHYSICAL & THERMAL PROPERTIES OF FUSED QUARTZ / SILICA
Fused quartz and silica have an extremely low coefficient of thermal expansion which imparts a high resistance to thermal shock. QSI fused quartz products can be heated to 1500 degrees C, or higher, and immersed in cold water without resultant physical damage. Annealing point is 1140 degrees C, strain point is 1070 degrees C, fusion point is between 1700 and 1800 degrees C.
Devitrification of fused quartz does not usually occur at temperatures below 1150 degrees C under normal conditions. High temperatures accelerate devitrification particularly in the presence of surface contaminants such as alkaline solutions, salts and vapors. To prevent deposit of salts, fused quartz should be handled with gloves. Radioactive materials, hydrogen and certain metals also accelerate devitrification.
Devitrification can be identified, as fused quartz cools below 250 degrees C, by a hazy film caused by micro-fine cracks. Devitrification can be forestalled by washing the quartz surface in a 10% solution of hydrofluoric acid for several minutes at room temperature, followed by a series of rinses in distilled or deionized water, followed by rapid drying.
ELECTRICAL PROPERTIES OF FUSED QUARTZ / SILICA
Fused quartz and silica have excellent electrical strength that remains highly stable with temperature variation. Quartz resistivity (10 to 17th power) ohms/cc at 25 degrees C is in the order of (10 to 17th power) ohms/cc at 1000 degrees C.
Fused Quartz Typical Trace Element Composition (ppm by weight)